A low sound level in the interior of a vehicle contributes to a relaxed driving experience and thus to an increased passive safety of a motor vehicle. The basic physical principles of absorption and insulation are known for reducing airborne sound. An optimum solution, which may be different for each individual application, is the coupling of the two systems.
Suitable combinations of materials are used, for example, for vehicle carpets/floor paneling systems, bulkhead modules or in the roof liner, and essentially contribute to the reduction of the background noise in the interior of the vehicle.
Component parts that cause insulation have a closed structure, i.e., they have no transmission of sound or air; they are most effective in a mass-spring structure. In addition to a flexible heavy layer, such a system also comprises an elastic and soft absorptive underlayer. The elastic layer causes a partial vibration decoupling of the individual elements and thus a reduction of the background noise. The insulation is determined by the mass of the heavy layer and by the spring stiffness of an absorptive layer.
However, a good sound absorption is necessary not only in vehicle construction. It is also important in aerospace technology, in rail transport, in the production of domestic appliances, or in house construction. A sound absorber for such applications must be optimized in such a way that sound is absorbed in a particular frequency range, which varies depending on the application, by appropriately selected materials.
In the prior art, sound absorbers made of fibrous materials, which are employed in a bonded form, for example, as non-wovens, are known, especially from the motor vehicle field.
Polymers, inorganic or metallic fibers in addition to natural fibers are employed as the fibers. Thus, EP 1 182 087 A2 describes an absorber comprising cellulose fibers and a synthetic resin. EP 0 909 680 A1 employs polyester fibers as sound absorbers. The diameter, thickness and drag of the fibers employed determine the efficiency and the frequency range in which the sound is absorbed.
DE 20 2004 009 726 U1 comprises a sound-absorbing self-supporting bulkhead cladding for motor vehicles for shielding the passenger compartment towards the engine compartment, comprising a sound absorber formed from a non-woven material pressed at least in portions thereof, characterized by being provided with a plastic frame formed by back injection or back molding on the pressed non-woven material and having at least one integrated fastening means.
The same or similar component parts made of foams, preferably PUR or melamine, with different densities instead of fibrous materials are also known.
The structure described as a spring-mass is known from the literature with different materials for the heavy layer (PUR, PE/EVA, EPDM and generally TPO based heavy layers), which are deep-drawn as a sheet, foamed, injection molded, sprayed, non-woven-laminated, or back foamed.
A lightweight sound-insulating cladding for a body part of a motor vehicle and a process for the production thereof are described in DE 10 2007 020 832 A1. The cladding, which is described, in particular, in the form of a lightweight bulkhead cladding, comprises a sound absorber layer, an essentially airtight sound insulation layer directly bonded to said sound absorber layer, and an adjacent foam layer, said sound absorber layer being formed from a porous absorber, preferably a non-woven or foam. The sound insulation layer consists of an integral skin layer of the foam layer having a thickness of at least 0.5 mm, which is bonded with the porous absorber by an integral joint, namely by back foaming the porous absorber, essentially without foam penetration.
A vehicle interior part made of a plastic material that also has a sound insulation is known from DE 34 48 259 C2. The plastic interior part includes polyolefin and/or polystyrene, synthetic rubber, and a filler.
If a foam is used as the sound absorber, preformed component parts are often prepared in this case too. These parts are optimized in terms of composition to exhibit the frequency-related sound absorption required for the corresponding position in the vehicle.
DE 20 2004 008 165 U1 describes a sound-insulating component part made of a highly filled thermoplastic elastomer based on styrene. The sound-insulating component part is suitable, in particular, for transmission tunnels and/or bulkhead claddings and additionally contains fillers.
A multilayer absorber according to the acoustic spring-mass system is described in DE 199 09 046 A1. The heavy layer serving as the mass is applied in different layer thicknesses and/or with different masses per unit area in situ to the porous soft layer serving as the spring, especially it is sprayed or injected thereto, or laid down through a flat sheet die.
DE 10 2004 054 646 A1 relates to a lightweight sound-insulating cladding for a body part of a motor vehicle, especially in the form of a bulkhead cladding, and to a process for producing such a cladding. The cladding includes a foam-molded sound insulation layer of polyurethane foam and a foam-molded sound-absorbing layer of open-cell polyurethane flexible foam.
A sound insulation part, especially for motor vehicles, comprising at least one sound-absorbing layer and at least one heavy layer bonded with it directly by an integral joint, and processes for producing it are described in DE 10 2005 056 840 B3. In a corresponding sound insulation part, the heavy layer is designed as a microporous sprayed skin to have at least one air-permeable part-surface zone and at least one air-impermeable part-surface zone.
DE 10 2004 037 767 A1 describes a sound insulator for a passenger compartment of a vehicle. In a corresponding sound insulator comprising an air cushion consisting of a damping sleeve made of an elastic material between a sound-emitting component part and the interior compartment, a sound insulating layer adapted to the shape of the component part and acting as a spring-mass system, which is sealed with a heavyweight film that forms a first shell of the air cushion on the side facing the interior compartment, is provided, and a second shell of the air cushion consisting of an elastic, preferably preformed, sheet sealed together with said heavyweight film is provided, wherein an air cushion is provided between said first and second shells.
DE 103 24 257 B3 comprises a sound absorber consisting of two interconnected textile non-woven fabrics with a thermoplastic and/or heat setting bond. The two textile non-woven fabrics have different layer thicknesses and densities to improve sound absorption.
A special form of absorbers are open two-layered absorbers in which the drags and thus the impedance of both layers differ.
In general, this stands for the combination of two layers; non-woven/non-woven, non-woven/foam and foam/foam with different drags are known. The acoustic performance is based on a balance between absorption and insulation, see, for example, DE 103 24 257 B3; EP 0 934 180 B2 (WO 98/18657); WO 98/18656; U.S. Pat. No. 6,145,617; WO 99/44817.
In DE 197 54 107 C1 and the prior art references therein, the absorption behavior of microperforated component parts is examined. What is examined here are baffle structures made from layers of microperforated sheets, pending from a ceiling or a roof as compact absorbers. The microperforated sheets are suitable for absorbing sound waves incident from one or both sides, vertically, obliquely or grazingly from the ambient very efficiently, especially for higher frequencies.
Component parts for use in motor vehicles that, in addition to one or two microperforated sheets, with fibrous or foamy absorbers are provided behind a sheet or between the two sheets are known from the prior art; the following documents may be mentioned by way of example: DE 100 22 902 A1, DE 10 2005 041 707 A1, DE 296 23 602 U1, DE 196 33 839 C2, EP 0 751 044 B1, EP 0 439 432 A1, EP 1 062 124 B1, WO 99/46147, EP 1 188 547 A1, DE 198 49 366 A1, DE 199 32 175 A1, DE 199 20 969 B4, DE 199 20 969 A1, EP 1 161 360 B1, WO 00/68039, DE 299 15 428 U1, DE 299 15 429 U1.
The use of a non-woven layer for sound insulation is further known from DE 10 2007 036 952 A1. The sound insulation described here comprises a foam layer and a barrier layer (absorption mat), which is characterized in that an optionally microperforated non-woven layer, a microperforated film, or a microperforated non-woven film is located partially or fully between the foam layer and the barrier layer.
A sound absorber may also have a foam layer instead of a non-woven layer. This is described, for example, in DE 100 22 902 A1. The covering element or molded element for means of transport as described herein comprises at least one microperforated film absorber, at least one foam and/or non-woven absorber and/or a clearance at a distance to a reverberant wall.
These component parts can be optimized for the corresponding frequency in terms of maximum absorption.
Absorbers are known to have a low insulation when the absorption is correspondingly high. A low absorption is found, in particular, in the technically interesting frequency range of from 500 to 2500 Hz. The absorption is reduced to the extent that the insulation is increased by applying the mass.
This often involves the need for an additional mold for molding the additional mass, which of course adds to the cost for the component part.
Almost all components are true-contoured with respect to the car body and thus cause high cost for the mold.